In the pulp and paper industry, logs are generally debarked before chipping. With some of logs which are to be used as raw material, the bark adheres very tightly to the logs or the bark is easily loosened as larger pieces, so that normal drum debarking is not fast enough, or in some cases discharging of the bark creates a problem. Particularly in connection with many tropical wood species, such as acacia and eucalyptus, the drum debarking method becomes problematic, as the long barks create problems by forming "bark balls" in the drum. The bark of acacia and eucalyptus also tends to loosen as long strips, so that the fresh bark also requires pretreatment, in order to avoid problems after contact with the drum. The long waste barks are not easily discharged through the bark slots of the drum and tend to come out along with the logs to the feeding conveyor of the chipper. In some cases, the drum debarking has also proven to be inefficient for the debarking of birch.
Other methods than drum debarking are used for debarking logs. A disadvantage of those methods is, however, their small capacity, which thus requires several debarking lines and a great number of devices. One known method of this kind is described in U.S. Pat. No. 4,685,498 of Fuji Kogyo. In this patent, the logs are transported through a channel-formed section in the longitudinal direction. On the bottom of the trough-shaped section, there are rollers disposed in longitudinal direction. When the rollers rotate, their teeth rasp the bark from the surface of the log.
In addition, debarking devices equipped with moving bottom elements have been tested. It has been shown in tests that a debarking apparatus shown in accordance with patent Finnish Patent No. 29866 does not rotate the logs efficiently enough, because the push force of the feed motion of the moving elements transferring the logs is not sufficiently stronger than the corresponding force in the reverse motion.
Periodically operating pretreatment devices are also being developed, the biggest disadvantage thereof being the constant need for supervision.
One such method is to break the surface of the bark with a high-pressure water jet. In such a procedure, however, a further method is also required in order to finally remove the bark. A known log pretreatment method of this type has been described in U.S. Pat. No. 4,180,109. In order to achieve the required water pressure, a strong pump is needed in this method. In addition, there are extreme requirements for the purity of the water so utilized.